1. Field of the Invention
The present invention relates to a method of growing large thin film single crystals and, more specifically, to a method of growing thin film crystals on substantially inert or amorphous substrates.
2. Prior Art
The use of single crystal semiconductor thin films in the manufacture of microelectronic thin film circuits has been severely hampered because of the limited area over which such single crystalline thin films can be grown. As a result, for many applications, the circuit designers must settle for a polycrystalline film which gives much poorer electrical performance and often precludes the device for many applications. Prior art methods of growing epitaxial layers are generally limited to growth upon a single crystal substrate whereby the area of the single crystal substrate is the maximum area to which the epitaxial layer may be grown. Because there is a maximum crystal size to which a single crystal substrate can be economically produced, the size of the epitaxial layer is also limited. Additionally, prior art growth methods require that the substrate have a temperature resistance at least equal to the crystallization temperature at which the growth takes place. Additionally, where a vapor growth process is used, the substrate must often withstand a chemically corrosive atmosphere at high temperatures. Finally, while epitaxially grown films offer a flexible method of obtaining impurity concentration profiles, as opposed to diffusion techniques, it is extremely difficult to grow or deposit such films through a mask in the desired pattern. Thus, to obtain a pattern from epitaxial growth, additional etching steps must be employed.
It has generally been accepted, that to grow single crystal films upon a single crystal substrate, the material must be deposited at a temperature above the epitaxial crystallization temperature so that the successive atoms being deposited may continue to form a growth pattern as a single crystal film.
Deposition below this temperature was found to result in either an amorphous film if deposited at low temperatures or a polycrystalline film if deposited at an intermediate temperature below the epitaxial crystallization temperature. It was thus believed that in heating an amorphous film, a polycrystalline film would be produced prior to the formation of a single crystal. Although Krikorian, ("Sputtering Parameters Affecting Epitaxial Growth of Semiconductor Single-Crystal Films", Single-Crystal Films, M. H. Francombe, H. Sato, Pergamon Press, Oxford, 1964, p. 113) experimentally located a "triple point" sputtering temperature at which amorphous, single crystal and polycrystalline films could be formed, only be extrapolation of the results was it suggested that an amorphous film could be directly converted to a single-crystal at relatively low temperatures.
Accordingly, since the polycrystalline intermediate phase has generally been assumed to exist below the epitaxial crystallization temperature, alternate schemes have required high temperature melting of the polycrystalline phase to form a single-crystal. U.S. Pat. No. 3,336,159 discloses the deposition of polycrystalline film onto what may be an amorphous substrate. This is followed by the heating of a single crystal in the polycrystalline film and then increasing the radius of the heating pattern so as to propagate that crystal throughout the film while melting all other crystals. Inherent in this process are the high melt temperatures as well as the requirement of selecting a suitable crystal in the polycrystalline film, carefully focusing the heat precisely around the crystal and radiating the heat outward so as to propagate that crystal and melt adjacent crystals to eliminate the danger of competing crystal growths. Where the beam is not precisely focused, more than one crystal may propagate, resulting in another polycrystalline film. Thus, it can be seen that great care must be taken in selecting the crystalline site, precisely measuring its position and dimension, and then precisely focusing the heat around it in order to generate a single crystal film.